The present invention relates to a fixed torque spring clutch and a winder having a fixed torque spring clutch.
Various types of fixed torque clutches have been known. Their structure must be complex in order to produce fixed torque accurately.
A conventional spring clutch is shown in FIG. 8. A coil spring 14 is wound on a rotary shaft 30. The rotation of the rotary shaft 30 in the direction of tightening the coil spring 14 is prevented; the rotation in the counter direction is allowed for fixed-torque rotation.
A reel base of a cassette tape recorder is shown in FIG. 9. A head-cap 22 and a gear 23 for fast forwarding tape are assembled and fixed on a rotary shaft 24 into which a fixed shaft is inserted. The rotary shaft 24 is inserted through a gear 25, which is transmitted driving force from a motor built in a tape recorder proper, with a play, and the gear 25 is pressed to contact a felt 28 and the gear 23 by a spring 27.
While recording and playing the tape recorder, the gear 25 is rotated at fixed speed to rotate the head-cap 22, the felt 28 and the gear 23 and a tape is wound round a reel of a cassette. The tape is advanced at fixed speed by a pinch roller, so that the gear 25 is rotated at fixed speed so as not to slacken the tape when the tape starts to move when no tape is wound round the reel of the cassette.
When the winding load becomes greater than the frictional force among the felt 28 and the gears 23 and 25, the gear 25 slips on the felt 28, so that unnecessary tension does not work on the tape and the tape can be wound without slackening.
The spring clutch shown in FIG. 8 is widely used in many fields because it has a small number of parts and simple structure.
The torque characteristic of the conventional spring clutch depends on the winding number of the coil spring 14, the diameter (inner diameter) of the coil spring with respect to the rotary shaft, etc.
In the manufacturing process of the coil spring, it is not easy to accurately wind with uniform diameter. Therefore, the spring clutch having this coil spring can be used with fixed torque but it is practically impossible to unite the torque characteristic of many spring clutches, so that many spring clutches below standard are produced.
In the reel base, in which a fixed torque is obtained by dynamic friction among the felt and other members, shown in FIG. 9, the coefficient of friction changes with the temperature and the humidity, so that the slip dynamic torque is unstable; the coefficient of friction also changes with abrasion of the felt, so that the slip dynamic torque also changes with the abrasion. Further, the coefficient of friction is sometimes changed by oil lubricated to the rotatable section, slidable section, etc. Additionally, there is no means for adjusting the slip dynamic torque, so that there is a possibility of manufacturing many clutches below the standard, and manufacturing cost including costs for parts and assembling is raised because of the complex structure.